Gravity is the answer. It's just that there is the possibility for gravity to, at long distances, drive things apart. This is either due to a purely mathematical feature of gravity or is it is because of the way that the energy of empty space interacts with gravity. We are not sure which yet and we may never be sure.

Gravity is the answer. It's just that there is the possibility for gravity to, at long distances, drive things apart. This is either due to a purely mathematical feature of gravity or is it is because of the way that the energy of empty space interacts with gravity. We are not sure which yet and we may never be sure.

If there is no "Negative Gravity", what is accelerating expansion of the Universe? I have tried to find an answer to this but have been unable to do so as yet. Thanks for comments.

Dark energy1, Joe. If you read up on Einstein, you'll comes across the stress-energy tensor2. This is a way of describing the distribution of energy in a region of space, usually when talking about a gravitational field3. His use of stress-energy was no accident, so look at say http://www.geol.umd.edu/~candela/press.html and understand that stress and pressure are very close - stress is directional, pressure isn’t, but they’re both measured in Pascals. Einstein was talking about the innate stress-energy of space, stress is akin to pressure, and you know that things that are under pressure will expand unless constrained. The universe is comprised of space, it isn’t thought to be infinite in extent, and it is thought to be “flat”. That means there’s no stress-energy beyond it, and no overall curvature, so there's nothing to hold it in. Hence the universe expands. The best everyday analogy I can think of is a stress ball. Squeeze it down in your fist and then let go.

1: There is an issue called “the vacuum catastrophe” which relates to the distinction between dark energy and vacuum energy and also the strong force.

2: A tensor is basically a matrix.

3: See Einstein's The Foundation of The General Theory of Relativity and look at page 185 where he says "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". This means there’s more energy per unit volume in a region of space which we describe as exhibiting a gravitational field, than in the surrounding space where the gravitational field is so attenuated as to be undetectable.

Einstein was talking about the innate stress-energy of space, stress is akin to pressure, and you know that things that are under pressure will expand unless constrained.

This is merely a pseudo-scientific explanation. The "pressure" terms in the stress-energy tensor (and only in spherically symmetric solutions) are not literally pressure. With one exception, these pressure terms contribute to attraction of one mass to another, or to the contraction of space in a homogeneous and isotropic model. That one exception is the so-called pressure terms associated with vacuum energy.

The cosmologists are still speculating, AND still making measurements. The measurements of the moment seem to indicate an accelerating expansion of the Universe; future measurements (which might encompass additional factors not-yet-thought-of) could possibly indicate a different conclusion. We shall see. At the moment therefore, your guess might be as good as anyone else's.

So, here is a guess, first proposed by the late Dr. Robert L. Forward. It starts with the Question of, "Where did the mass-energy of the Universe come from, without violating the Law of Conservation of Energy?". The proposed answer is that the Universe consists half of ordinary mass-energy and half of "negative" mass-energy. This stuff is not impossible in General Relativity; it would simply be associated with "curved space" that has the opposite curvature than that which is associated with ordinary mass-energy. Negative mass-energy would also have a set of properties that, compared to itself, could be very similar to the properties of ordinary mass-energy (compared to itself). It could gravitationally attract itself, for example, but showing why it might be able to do that will require a bit of explaining.

First, though, let me make very clear that the stuff known as "antimatter" cannot possess negative mass-energy. That's because physicists on Earth have been making lots of particles of antimatter for decades, and always have been making them from ordinary energy, per Einstein's famous equation E=mc2. Half of that ordinary energy became ordinary matter, and the other half became antimatter. Therefore antimatter consists of ordinary energy, period. (Note this means that if matter can exist having negative mass-energy, it might be able to exist in two forms, one being the equivalent of ordinary matter, and one being the equivalent of antimatter. I'm only going to talk about the ordinary-matter equivalent, below.)

The explanation begins with Planck's Constant and the equation that describes the energy of an ordinary photon (the first one on that linked page). Because the energy of an ordinay photon is positive, Planck's Constant is associated with positive energy, per the rules of "Dimensional Analysis". This means that if we want to write the equation that describes a photon that has negative energy, Planck's constant, in that equation, must also be associated with negative energy (it must be a negative number, not a positive number). This is a very important point. Most of the time, when physicists have deigned to speculate about negative mass-energy, they (including Dr. Forward) have failed to include that point, about using a negative Planck's Constant in their speculations. The net result is that they would say that when interacting gravitationally, two objects having negative mass-energy would exhibit acceleration-away-from-each-other. However, if we are ever one day to describe Gravitation using Quantum Mechanics, at that time Planck's Constant will become an integral part of that description (most probably buried somewhere inside "G", the gravitational constant). This means that if we use those Q.M. equations to describe negative mass-energy and its behavior, we will need to use the negative version of Planck's Constant (and therefore a negative "G"), for those equations to be self-consistent. And one result will be, two objects having negative mass-energy will exhibit acceleration-toward-each-other, just like two objects having ordinary mass-energy.

The fun begins when we start to think about gravitational interactions between one object that has ordinary mass and one that has negative mass. Which version of Planck's Constant should we use? I have no idea (maybe both!)! There is a chance that the two objects would exhibit acceleration-away-from-each-other. If so, then, and if half of the Universe consisted of negative mass-energy, per Energy Conservation, then we could expect the ordinary mass to accelerate toward itself, the negative mass to accelerate toward itself, and the two different types to accelerate away from each other. The net result would look a lot like today's Universe. The obvious "clumps" are places where ordinary mass has accumulated, and the "voids" are the places where negative mass has invisibly clumped (our instruments are not designed to detect photons of negative energy, and that's why the "voids" look like voids, despite the presence of superclusters of galaxies of stars that are fusing "negative hydrogen").

It remains an interesting question, regarding the Universe as a whole, whether or not the current distribution of ordinary and negative matter can result in an overall increased expansion. Remember all the ordinary matter is attracting itself, and all the negative matter is acting equivalently. This should balance any repulsion between the two. So, we could not expect an overall acceleration. On the other hand, we also could not expect the original rate of expansion of the Universe (after the "inflation" period, I'm talking about here) to have ever slowed down! This is why it is critical to obtain more observational data. The observed speeds of galaxies do not necessarily need to be explained in terms of gravitational slowing (due to a Universe of ordinary mass only), followed by as-yet-unexplained acceleration. The matter in the Universe could have been going that fast all along.

Joe: check my answer out elsewhere. You might find people saying they wouldn't express it in quite the same terms but it's a reasonable reasoned answer.

I also urge this. If you are truly interested in gravitational physics, then hit the books. You will find far more satisfaction than hand-waiving answers. You will definitely find that Farsight's answer has no content.

Joe: check my answer out elsewhere. You might find people saying they wouldn't express it in quite the same terms but it's a reasonable reasoned answer.

I also urge this. If you are truly interested in gravitational physics, then hit the books. You will find far more satisfaction than hand-waiving answers. You will definitely find that Farsight's answer has no content.

Hi, PhysBang. I know the conventional answer but I just do not believe it. I hope that doesn't get me shot for having a difference of opinion. I am not trying to convince others of my opinion. Thanks for comments. Joe L. Ogan

Yes Joe, the ide of dark energy having a negative pressure might make sense mathematically but I have yet to see a mathematician translate into words how this negative energy 'pushes' the space lanes wider :) as we old, ever so salty, spacedogs say ::))dark energy and matter

One possibility might be some sort of topological 'defects' that we don't know of, or maybe it has to do with times arrow, if now 'space itself', a vacuum that is, can vary, why shouldn't the arrow possibly also be able to vary in some unknown way?

Although, if it does, then we can't be sure of anything as redshift too would become susceptible if so. It may be so that the vast empty stretches of the vacuum (space) act in a manner unknown to us, which by the way would make sense to me, but then I would expect all properties of SpaceTime to be similarly affected, not only space.==

Here is a recent idea where mathematicians/physicists have made an innovative approach to what it can be, and for once also made it more or less testable. The Day The Universe Froze

You know Joe, one of my beliefs is that SpaceTime is a 'whole' thing. To me that idea means that if you change f.ex speed of light you will also need to change all other constants. If it was proved that light truly have changed its velocity in a vacuum with other constants staying the same I think Einsteins SpaceTime would be threatened as those factors like lightspeed mass etc all go into each other.

And no matter what people says Einsteins model have been tested experimentally, over and over without anyone being able to prove it wrong. I'm not discussing QM now btw. So for your, or razatlaby's, idea to work you will have to factor in some changes in those other constants too, as you otherwise will have to create a totally new cosmology. If we assume f.ex that 'times arrow' runs slower locally when there is no or very little matter present, like in deep space, then that would contradict all we know about how 'times arrow' slows down with mass or acceleration (as seen from inside such a 'frame', when comparing it against a 'stationarity observer' in another 'frame of reference')

We know that mass and times arrow works together, we also know, or at least have strong theoretical evidence that uniform acceleration can do strange and remarkable things with times arrow like showing us phenomena not belonging to our ''arrow of time' as seen from an inertial frame (Earth) suddenly existing observably inside that 'arrow of time'. Like 'virtual particles' suddenly becoming as 'real and existing, same as all other light particles'. How this last can be explained I still don't have a grip on, but what it all implies is that you can't just lift out one thingie from SpaceTime and manipulate that one, while expecting the rest of SpaceTimes properties to stay the same.

So to make an idea like light slowing down work, you both have to decide that SpaceTime then is a puzzle of different things, where the jigsaw somehow have fallen into place by 'itself', and then that Einsteins ideas was wrong too. All together those two ideas will bring us back around a hundred years as we now will have to build a totally new ground for everything. From 'black body radiation' to all those constants and ideas we use, as nothing built on the assumption that 'c' is a constant then will be correct. That means that whoever it is doing this kind of things will have to redefine Planck's lengths/times/size etc etc. So he not only need to be new Einstein, he will also need to be Planck, Maxwell, Heisenberg, etc. As I see it that is:)

The real solution will be accepting the universe Einstein shows us, as well as the universe QM speaks about but relate it in such a way that they become subsets of a 'grander solution' just as Newtons universe is a subset of Einsteins. That may also mean that our next step becomes a blend of thing we already know but haven't put together yet.

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